A lithography system is a system that applies a desired pattern onto a target portion of a substrate surface, for example, in the manufacture of an integrated circuit (IC). A lithography system employs a patterning structure, which is alternatively referred to as a mask or a reticle, that is used to generate a circuit pattern corresponding to an individual layer of the IC, and its pattern is imaged onto a target portion (e.g., including part of, one or several dies) on the substrate surface or wafer. The substrate surface has a layer of radiation-sensitive material (resist) disposed thereon. The patterned substrate is then developed, and one layer of the circuit pattern appears. When repeated a number of times, the process results in a fully patterned integrated circuit substrate.
Lithography systems, such as electron beam lithography systems, have a limited depth of focus. Thus, variation of height of the writing surface may result in out-of-focus imaging, which may induce control errors. The height of a substrate surface within a lithography system may vary across the substrate. Not only are there variations in substrate thickness, but also there may be particles between the substrate and its support stage, causing the substrate to deform when held in the stage. If these height variations are not corrected, the surface of the substrate being exposed by the writing beam (for example, the electron writing beam), may be beyond the depth of focus of the optical system, causing blurring of the image of the projected pattern. The substrate may also be tilted with respect to the lithography system, and more particularly, with respect to a nominal or ideal writing plane of the lithography system. This tilt may also appear as an optical de-focusing that varies across the substrate surface.
Various approaches have been disclosed in the art for monitoring and measuring height and/or tilt variations. For example, reference U.S. Pat. Nos. 5,894,132; 6,884,554; 7,072,024; and 7,265,364. Although certain ones of the monitoring approaches described in these Letters Patents do provide a mechanism for ascertaining tilt of a substrate within a lithography system, the approaches are generally complex and/or difficult to align and calibrate, and may have a lower sensitivity than desired, or overly rely on exact positioning of the support stage holding the substrate.